You are probably going to get about one answer for each person that posts. I have the MS Intellimouse with the Intellieye, but others claim that the light cannot track fast movements well enough for FPS gaming (then how do I consistently end up at or near the top when I play? hmmm...). I think it depends on style of play for that preference.
I have used both a Kensington scroll mouse and the MS Intellimouse. The MS mouse is better. I have not used Logitech or the MS Explorer or the Razor Boomslang, which some people love. It comes down to personal preference, really, but you do need the ability to set the sampling rate of the PS/2 port to increase senstivity (that does help). Also, when you change mice, it will take some adjustment before you become proficient with a new one.